I suppose americans might be less familiar with this phenomenon because you have mailboxes outside the house instead of as slots built into your doors? Here in the UK it is most common for doors to have a letter shaped postal slot on the door. This size:
Extremely common for houses to burn down in this way.
https://www.liverpoolecho.co.uk/news/liverpool-news/firework-thrown-house-gang-sparks-30148944
https://www.itv.com/news/london/2023-04-28/teenagers-jailed-after-woman-88-killed-by-firework-stuffed-through-letterbox
https://www.essexlive.news/news/essex-news/son-essex-woman-killed-after-9684214
I suppose americans might be less familiar with this phenomenon because you have mailboxes outside the house instead of as slots built into your doors? Here in the UK it is most common for doors to have a letter shaped postal slot on the door. This size: